Method of manufacturing solid electrolytic capacitor

Abstract: The invention provides a method for manufacturing a solid electrolytic capacitor having oxide film (4) of valve action metal (1) as a dielectric substance and conducting polymer as solid electrolyte. The method includes the steps of forming solid monomer compound on a surface of the oxide film (4) by applying monomer compound solution to the surface and then drying the monomer compound solution, and forming a conducting polymer layer (5) by polymerizing the solid monomer compound with an oxidizer. The method provides a capacitor having a quite small impedance in a resonant frequency and also having a superior high frequency characteristic.

Description: BACKGROUND OF THE INVENTION The invention relates to a method for manufacturing a solid electrolytic capacitor having electrically conductive high polymer as a solid electrolyte, and more particularly to a method for manufacturing such a solid electrolytic capacitor having a superior frequency characteristic. In general, a solid electrolytic capacitor includes a first electrode or an anode composed of porous compact of valve action metal such as tantalum (Ta) and aluminum (Al), a dielectric substance composed of oxide film of the anode, and a second electrode or a cathode composed of solid electrolyte such as manganese dioxide (MnO 2 ) and 7,7,8,8-tetracyanoquinodimethane (TCNQ) complex salt. The solid electrolyte is required to have functions for electrically connecting an entire dielectric substance disposed in the porous compact to leads of the electrodes, and for repairing electrical short-circuit derived from defects of the dielectric oxide film. Accordingly, it is not allowed to use metal having high electrical conductivity but having no function for repairing electrical short-circuit as a solid electrolyte. For instance, manganese dioxide (MnO 2 ), which can be converted to insulative material due to heat generated by short-circuit current, has been often used as a solid electrolyte. However, a solid electrolytic capacitor having manganese dioxide (MnO 2 ) as a part of an electrode cannot have a sufficiently large electrical conductivity, and hence has to have a large impedance in high frequency band. In addition, such a solid electrolytic capacitor can have only a poor thermal resistance, because TCNQ complex salt is easy to be thermally decomposed. Recently, new materials have been developed in the field of high polymer. For instance, there has been developed a conducting polymer including conjugate high polymer such as polyacetylene, polyp-phenylene, polypyrrole and polyaniline to which is added a dopant or an electron donative and/or electron attractive compound. For instance, Japanese Unexamined Patent Public Disclosure No. 64-24410 has suggested a method for manufacturing a conducting polymer layer by introducing monomer of conducting polymer into a surface of oxide film which plays a role of dielectric substance in liquid phase, and then adding thereto an oxidizer solution including an organic acid. On the conducting polymer layer are formed electrodes each composed of a graphite layer and a silver paste layer. Japanese Unexamined Patent Public Disclosure No.2-74019 has suggested a method of manufacturing a composite layer composed of a high polymer layer and a conducting polymer layer, including the steps of covering oxide film with high polymer film in which an oxidizer is distributed in advance, and contacting the oxide film with monomer of conducting polymer. In the aforementioned method for manufacturing a solid electrolytic capacitor, suggested by Japanese Unexamined Patent Public Disclosure No. 64-24410, when the liquid monomer of conducting polymer formed on oxide film is made to contact with an oxidizer solution, the liquid monomer is diffused in the oxidizer solution. Hence, a required amount of conducting polymer cannot be produced. Thus, even if the method is applied to an existing capacitor which utilizes oxide film having a greatly enlarged surface, the capacitor can have disadvantageously only a low capacity attainment rate which is defined an actual capacitance value relative to a designed value. On the other hand, in the method suggested by Japanese Unexamined Patent Public Disclosure No. 2-74019, since an oxidizer is in advance diffused in high polymer film, there would not occur a diffusion of the monomer into an oxidizer. However, the composite layer formed on the oxide film is composed of originally non-conducting polymer film and a conducting polymer, and hence the method has a problem that a resultant capacitor can have only quite a poor frequency characteristic. SUMMARY OF THE INVENTION In view of the foregoing problems of the conventional methods, it is an object of the present invention to provide a method for manufacturing a solid electrolytic capacitor, which method is capable of achieving a high capacity attainment rate even for a capacitor utilizing oxide film having a greatly enlarged surface, and also capable of providing a superior frequency characteristic particularly in high frequency band. The invention provides a method for manufacturing a solid electrolytic capacitor having oxide film of valve action metal as a dielectric substance and conducting polymer as solid electrolyte, the method including the steps of forming solid monomer compound on a surface of the oxide film by applying monomer compound solution to the surface of the oxide film and then drying the monomer compound solution, and forming a conducting polymer layer by polymerizing the solid monomer compound with an oxidizer. In a preferred embodiment, the monomer compound solution has a concentration of at least 1% by weight. In another preferred embodiment, the method further includes the step of increasing a surface area of the valve action metal prior to forming the solid monomer compound on a surface of the valve action metal. In still another preferred embodiment, the surface area of the valve action metal is increased by forming a mass of irregularities on the surface of the valve action metal. In yet another preferred embodiment, the oxide film is formed by means of anodic oxidation. In still yet another preferred embodiment, the oxide film is formed over the irregularities by means of anodic oxidation. In further preferred embodiment, the conducting polymer layer is formed so that the layer deeply penetrates the irregularities. In further preferred embodiment, the method further includes the step of forming a cathode on the conducting polymer layer oppositely to the solid monomer compound. In further preferred embodiment, the method further includes the step of forming a carbon layer between the conducting polymer layer and the cathode. In further preferred embodiment, the carbon layer is a graphite layer. In further preferred embodiment, the method further includes the step of sealing a resultant with epoxy resin. The invention further provides a method for manufacturing a solid electrolytic capacitor having oxide film of valve action metal as a dielectric substance and conducting polymer as solid electrolyte, the method including the steps of anodizing a pellet composed of sintered tantalum powder, drying the anodized pellet, immersing the pellet in aniline salt solution, drying again the pellet, immersing the pellet in oxidant solution, polymerizing the pellet to form oxide film composed of polyaniline on the oxide film, immersing the pellet in acid solution and then drying, and repeating third to seventh steps in a desired number of times. In a preferred embodiment, the method further includes the step of applying carbon paste to a resultant subsequently to the step of repeating third to seventh steps. In another preferred embodiment, the method further includes the step of applying silver paste to a resultant subsequently to the step of applying carbon paste. The advantages obtained by the aforementioned present invention will be described hereinbelow. The invention provides advantages that a capacitor having a greatly enlarged surface can be manufactured, for instance, by forming a mass of irregularities on a surface of a conducting polymer layer, and that a solid electrolytic capacitor having a superior high frequency characteristic can be easily manufactured. Comparing to a conventional method in which liquid monomer is merely polymerized with an oxidizer on oxide film, the method in accordance with the invention has advantages over the conventional method that the method can manufacture a capacitor having a high capacity attainment rate even from a capacitor having an enlarged surface. Comparing to a conventional method in which insulative high polymer containing oxidizer is formed on oxide film and then conducting polymer is formed, the method in accordance with the invention forms conducting polymer, which has a high electrical conductivity, directly on oxide film, and hence the invention provides advantages over the conventional method that the invention provides a capacitor having a quite small impedance in resonant frequency and also having a superior high frequency characteristic. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a flow chart showing the steps of the method in accordance with the invention. Fig. 2 is a schematic cross-sectional view of a solid electrolytic capacitor manufactured in accordance with the embodiment of the invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments in accordance with the present invention will be explained hereinbelow with reference to drawings. With reference to Fig. 1, the method for manufacturing a solid electrolytic capacitor in accordance with the invention will be explained hereinbelow. First, in step 1, a mass of irregularities 2 are formed on a surface 1a of valve action metal 1 which will operate as an anode, in order to increase a surface area of the surface 1a of the valve action metal 1, as illustrated in a partially enlarged figure included in Fig. 2. Then, oxide film 4 operating as a dielectric substance are formed by means of anodic oxidation over the surface 1a of the valve action metal 1 on which the irregularities 2 are formed. Then, in step 2, monomer compound solution is applied to a surface of the oxide film 4. Then, the monomer compound solution is solidified by drying so that solid electrolyte 5 composed of the solidified monomer compound solution and operating as a conducting polymer layer deeply penetrates concave portions 2a of the irregularities 2. In step 3, the conducting polymer layer 5 is polymerized with oxidizer solution to thereby form a chemically oxidized, polymerized, conducting polymer film. In step 4, a metal electrode 7 operating as a cathode is attached to the conducting polymer layer 5. Between the cathode 7 and the conducting polymer layer 5 may be disposed a carbon layer or a graphite layer 6 for maintaining an appropriate contact of the cathode 7 with the layer 5. Then, as illustrated in Fig. 4, a pair of electrode leads 8 and 9 are formed protruding from the thus manufactured resultant 10. Finally, around the resultant 10 having the electrode leads 8 and 9 protruding therefrom is surrounded by epoxy resin 11. Thus, a solid electrolytic capacitor is manufactured. Sample 1 A pellet composed of sintered tantalum (Ta) fine powder and shaped in rectangular parallelopiped having 1 mm of length, 1 mm of width and 1 mm of thickness, and having 30,000/g of a CV product value which is defined as a product of capacitance µF per 1 gram and an anodic oxidation voltage V, was anodic-oxidized with a voltage of 48V in a solution containing 0.1 % by weight of phosphoric acid. Then, the pellet was washed and dried. Then, there was prepared aniline salt solution by mixing a solvent containing 70% by weight of ethanol and 30 % by weight of water with aniline and p-toluenesulfonic acid equal to aniline in an amount in mol so that the aniline and p-toluenesulfonic acid occupy 15% by weight on the basis of a weight of the solvent. Then, the sintered pellet was soaked in the aniline solution for 30 seconds while the pellet was maintained at 0 degree of temperature, and subsequently the pellet was stood for 10 minutes at a room temperature. As a result, aniline salt in solid was precipitated on a surface and inside of the sintered pellet. Then, the sintered pellet was soaked for 30 seconds at 0 degree of temperature in a solution containing 5% by weight of ammonium dichromate and 40% by weight of p-toluenesulfonic acid, and subsequently was stood for 30 minutes at a room temperature. Thus, the polymerization was achieved and hence black-colored polyaniline was formed on a surface of oxide film. Furthermore, the pellet was soaked for 5 minutes in an acidic solution containing p-toluenesulfonic acid by 7% by weight diluted with a solvent containing 50% by weight of ethanol and 50% by weight of water, and then was stood for 30 minutes at a room temperature. The steps of soaking the pellet in the aniline salt solution, soaking the pellet in the oxidizer solution, achieving the polymerization, and soaking the pellet in the acidic solution were repeated six times. Then, to a resultant were applied carbon paste commercially available from Acheson Industries, Inc. under the trade name of "Electrodug 112" and silver paste commercially available from Fujikura Kasei Corporation under the trade name of "Dotite D-550". Then, cathode leads were attached to the resultant and the resultant was entirely covered with epoxy resin. Thus, a solid electrolytic capacitor was completed. Sample 2 A pellet composed of sintered tantalum (Ta) fine powder and cylindrically shaped having 1 mm of diameter and 1 mm of height, and having 27,000/g of a CV product value was anodic-oxidized with a voltage of 13V in a solution containing 0.1 % by weight of phosphoric acid. Then, the pellet was washed and dried. Then, there was prepared aniline salt solution by mixing a solvent containing 70% by weight of methanol and 30 % by weight of water with aniline and p-toluenesulfonic acid equal to aniline in an amount in mol so that the aniline and p-toluenesulfonic acid occupy 15% by weight on the basis of a weight of the solvent. Then, the sintered pellet was soaked in the aniline solution for 30 seconds while the pellet was maintained at 0 degree of temperature, and subsequently the pellet was stood for 10 minutes at a room temperature. As a result, aniline salt in solid was precipitated on a surface and inside of the sintered pellet. Then, the sintered pellet was soaked for 30 seconds at 0 degree of temperature in a solution containing 15% by weight of ammonium peroxodisulfate and 15% by weight of p-toluenesulfonic acid, and subsequently was stood for 30 minutes at a room temperature. Thus, the polymerization was achieved and hence black-colored polyaniline was formed on a surface of oxide film. In accordance with the same fashions as those of the sample 1 except the aforementioned ones, a solid electrolytic capacitor was manufactured. Sample 3 In sample 3 was used a pellet identical to that used in sample 2. There was prepared aniline salt solution by mixing a solvent containing 70% by weight of ethanol and 30 % by weight of water with aniline and benzenesulfonic acid equal to aniline in an amount in mol so that the aniline and benzenesulfonic acid occupy 15% by weight on the basis of a weight of the solvent. Then, the sintered pellet was soaked in the aniline solution for 30 seconds while the pellet was maintained at 0 degree of temperature, and subsequently the pellet was stood for 10 minutes at a room temperature. As a result, aniline salt in solid was precipitated on [a surface and inside of the sintered pellet. Then, the polymerization of aniline was achieved in the same way as that of sample 1. Then, leads were attached to a resultant to thereby complete a solid electrolytic capacitor. The number of polymerization for obtaining sufficient thickness was six (6). Comparative Sample 1 In comparative sample 1 was used a pellet composed of sintered tantalum (Ta) fine powder and shaped in rectangular parallelopiped having 1 mm of length, 1 mm of width and 1 mm of thickness, namely, identical to that used in sample 1. There was prepared aniline solution by mixing a solvent containing 70% by weight of ethanol and 30 % by weight of water with aniline so that the aniline occupies 15% by weight on the basis of a weight of the solvent. Then, the sintered pellet was soaked in the aniline solution for 30 seconds while the pellet was maintained at 0 degree of temperature, and subsequently the pellet was stood for 10 minutes at a room temperature. However, unlike sample 1, there was not precipitated aniline salt in solid on a surface and inside of the sintered pellet. Then, the polymerization of aniline was achieved in the same way as that of sample 1. Then, leads were attached to a resultant to thereby complete a solid electrolytic capacitor. The number of polymerization for obtaining sufficient thickness was fifty (50). Comparative Sample 2 In comparative sample 2 was used a pellet composed of sintered tantalum (Ta) fine powder and shaped in rectangular parallelopiped having 1 mm of length, 1 mm of width and 1 mm of thickness, namely, identical to that used in sample 1. There was prepared aniline salt solution by mixing a solvent containing 70% by weight of ethanol and 30 % by weight of water with aniline and p-toluenesulfonic acid equal to aniline in an amount in mol so that the aniline and p-toluenesulfonic acid occupy 0.8% by weight on the basis of a weight of the solvent. Then, the sintered pellet was soaked in the aniline solution for 30 seconds while the pellet was maintained at 0 degree of temperature, and subsequently the pellet was stood for 10 minutes at a room temperature. As a result, quite a small amount of aniline salt in solid was precipitated on a surface and inside of the sintered pellet. Then, the polymerization of aniline was achieved in the same way as that of sample 1. Then, leads were attached to a resultant to thereby complete a solid electrolytic capacitor. The number of polymerization for obtaining sufficient thickness was fifty (50). Table 1 shows CV product values, a mixture rate by weight of the monomer solution, the concentration of monomer relative to a weight of solvent, the number of polymerization, the obtained capacitor attainment rate C/C 0 wherein C 0 indicates a capacity in electrolytic solution, leak current (LC), and a resonant frequency impedance (Z) each obtained in samples 1 to 3 and comparative samples 1 and 2. The table 1 shows that a solid electrolytic capacitor in accordance with the invention provides a large capacitor attainment rate even applied to a capacitor element having a greatly enlarged surface, and has only a small impedance in resonant frequency together with a superior high frequency characteristic.

Claims: A method for manufacturing a solid electrolytic capacitor having oxide film of (4) valve action metal (1) as a dielectric substance and conducting polymer as solid electrolyte, characterized by the steps of: forming solid monomer compound on a surface of the oxide film (4) by applying monomer compound solution to the surface of the oxide film (4) and then drying the monomer compound solution; and forming a conducting polymer layer (5) by polymerizing the solid monomer compound with an oxidizer. The method as recited in claim 1, wherein the monomer compound solution has a concentration of at least 1% by weight. The method as recited in claim 1 or 2 further comprising the step of increasing a surface area of the valve action metal (1) prior to forming the solid monomer compound on a surface of the oxide film (4). The method as recited in claim 3, wherein the surface area of the valve action metal (1) is increased by forming a mass of irregularities (2) on the surface (1a) of the valve action metal (1). The method as recited in any of the preceding claims, wherein the oxide film (4) is formed by means of anodic oxidation. The method as recited in claim 4, wherein the oxide film (4) is formed over the irregularities (2) by means of anodic oxidation. The method as recited in claim 4, wherein the conducting polymer layer (5) is formed so that the layer (5) deeply penetrates the irregularities (2). The method as recited in any of the preceding claims further comprising the step of forming a cathode (7) on the conducting polymer layer (5) oppositely to the solid monomer compound. The method as recited in claim 8 further comprising the step of forming a carbon layer (6) between the conducting polymer layer (5) and the cathode (7). The method as recited in claim 9, wherein the carbon layer (6) is a graphite layer. The method as recited in any of the preceding claims further comprising the step of sealing a resultant with epoxy resin (11). A method for manufacturing a solid electrolytic capacitor having oxide film (4) of valve action metal (1) as a dielectric substance and conducting polymer as solid electrolyte, characterized by the steps of: anodizing a pellet composed of sintered tantalum powder; drying the anodized pellet; immersing the pellet in aniline salt solution; drying again the pellet; immersing the pellet in oxidant solution; polymerizing the pellet to form oxide film composed of polyaniline on the oxide film; immersing the pellet in acid solution and then drying; and repeating third to seventh steps in a desired number of times. The method as recited in claim 12 further comprising the step of applying carbon paste to a resultant subsequently to the step of repeating third to seventh steps. The method as recited in claim 13 further comprising the step of applying silver paste to a resultant subsequently to the step of applying carbon paste.

Inventors: ARAI SATOSHI, KOBAYASHI ATSUSHI, ARAI, SATOSHI,, KOBAYASHI, ATSUSHI,, ARAI, SATOSHI, C/O NEC CORPORATION, KOBAYASHI, ATSUSHI, C/O NEC CORPORATION
Applicants: NIPPON ELECTRIC CO, NEC CORPORATION
Application year: 1994
Main classification: H01G9
IPC codes: H01G9